Rotary drum filters are well known and are used in the pulp and paper industry to filter and wash pulp slurries, for example, from U.S. Pat. Nos. 4,808,265 and 4,906,364 (the entire contents of which are expressly incorporated hereinto by reference). Rotary drum filters generally comprise a cylindrical drum mounted for rotation in a tank containing a slurry solution. A filter deck assembly is mounted radially outward of the surface of the drum. As the drum is rotated, a low pressure is maintained inside the drum so that a pulp sheet is formed on the surface of the filter deck assembly. The filter deck assembly functions to space the pulp from the drum surface to facilitate filtering and drainage.
The filter deck assembly usually includes a filter deck on top and a drainage deck below, which directs the filtrate to filtrate compartments. The liquid filtrate is drawn through the filter deck and the drainage deck and into the filtrate compartments by a pressure differential. Filtrate is removed from the filtrate compartments in a conventional manner. The filter deck assembly must prevent rewetting of the pulp sheet as the drum rotates through the descending path of the filtrate cycle.
Metal plates having fine perforations, slots or other apertures are often used as the filter deck. Corrugated sheets attached below the filter decks are used as the drainage decks. The metal plates, the corrugated sheets, and the drum may each be made of different metals.
The filter deck assembly for rotary drum filters must be capable of withstanding differential thermal expansion of the component elements due to the temperature variations in the filtering cycles. For example, temperature differentials of up to 100 degrees Fahrenheit expose the filter deck assembly to heavy thermal shocks once every cylinder revolution.
Unlike the deck assemblies known to date, the present inventive technology provides a filter deck assembly for a rotary drum filter that has a greater drainage surface, a more efficient filtering action, and is constructed and arranged to permit thermal expansion of the component elements without resultant damage. Furthermore, the present inventive technology provides a filter deck assembly that effectively prevents rewetting of the pulp sheet from a run back of the filtrate as the drum rotates through the descending path of the cycle. The present inventive technology also provides a filter deck assembly that is durable and has a long commercial life. Finally, the present inventive technology provides a filter deck assembly that is particularly constructed and arranged to allow simple and quick replacement of filter deck assembly sections.
The foregoing advantages are obtained in a rotary drum filter having a plurality of longitudinally and radially extending division grids mounted about the outer circumference of a drum. A plurality of corrugated sheets for drainage decks are provided, with the leading and trailing edges of each sheet mounted on circumferentially adjacent pairs of division grids to define filtrate compartments. An equivalent number of perforated filter decks or plates are also provided. Each filter deck or plate is attached to the top of a corrugated sheet (drainage deck) and has a leading edge that is set back from the leading edge of the attached corrugated sheet. An equivalent number of perforated cap strips (longitudinal cap strips) are attached to the tops of the division grids to keep the filter deck assemblies within their proper locations and ranges of movement, to allow maximum drainage and to allow ease of replacement of cap strips and filter deck assemblies.
A seal member is positioned in front of (partially or wholly) and/or underneath the leading edge, or in close proximity thereto, of each corrugated sheet (drainage deck). A seal clip is positioned and attached in front of and/or underneath the leading edge of each corrugated sheet and holds the seal member in place. The seal clip can be tack-welded or spot welded to the corrugated sheet, for example, and depending on the geometry, to the front and/or bottom of the corrugated sheet. When the seal member is positioned in front of and/or underneath the corrugated sheet, the seal member may be configured in a manner so that all or a portion of the seal member is located or pinched between the seal clip and the leading edge, or near the leading edge, of the corrugated sheet. The seal member is a compressible material that acts in conjunction with the seal clip to, among other things, seal the leading edge area of the drainage deck when the corresponding portion of the rotary drum is descending. It also assists with and maximizes the filter deck drainage area and deck pressure differential in the area of the seal clip.
Some of the problems with the prior art devices are the loss of drainage in the area corresponding to the seal member and the loss of pressure differential in that area. For example, the L-shaped seal member of U.S. Pat. No. 4,906,364 results in the loss of drainage in the area of the seal member and the loss of pressure differential in that area. The loss of drainage and pressure differential is detrimental to the capacity of the rotary drum filter and degrades washing efficiency. The present inventive technology provides additional drainage capacity and improved washing efficiency.